DUXBURY (CBS) – Swimmers were ordered out of the water at a Duxbury beach Monday after a large shark was spotted.

Friends of Samantha Mcgibbon and Cassandra Hickie planned a surprise sweet 16 birthday party on Duxbury Beach for both Pembroke girls. But when the group got to the beach Monday afternoon, the surprise was on all of them.

“We saw the police and the helicopter circling so we knew that something was going on,” Mcgibbon said. “I was definitely nervous, didn’t want to go in the water.”

“Yeah, we were all really scared because we’re all afraid of sharks,” added Hickie.

Around 1:30 Monday afternoon, a Massachusetts State Police helicopter spotted a lone great white shark estimated to be between 12-15 feet long in the waters not far from Duxbury’s Powder Point Bridge.

“Pretty soon after that the lifeguard got on the bullhorn and said everybody out of the water,” explained Duxbury resident Jack McWilliam. “I’ve been here for 30 years and to mind this has never happened before.”

Lifeguard Peter Emmett chose his words carefully.

“You can’t tell people shark in the water,” he said. “That’d be pandemonium.”

Only after they got everybody out of the water did officials tell them about the great white. Crews tracked the shark for more than an hour as it lingered about a hundred yards offshore. Eventually, they lost sight of it as it appeared to head off to deeper waters. Swimmers got the all clear around 4pm to go back in.

“We advised them just stay within knee-deep and swim at your own risk, really, because it could come back at any time,” explained Emmett.